1. Field of the Invention
The invention relates to a driving force transmission apparatus and a method of manufacturing the driving force transmission apparatus.
2. Description of Related Art Japanese Patent Application Publication No. 11-153157 (JP 11-153157 A) and Japanese Patent Application Publication No. 2004-116764 (JP 2004-116764 A) each describe a driving force transmission apparatus in which an electromagnetic clutch unit is used to control a driving force that is transmitted between an outer rotary member and an inner rotary member. The electromagnetic clutch unit includes an electromagnet and a magnetic path forming member. The magnetic path forming member forms a magnetic path in cooperation with an armature and clutch discs. The magnetic path forming member includes a portion that exhibits magnetic properties and a portion that exhibits non-magnetic properties.
JP 11-153157 A describes a method in which a non-magnetic material is joined to a magnetic material by welding to form a magnetic path forming member. JP 2004-116764 A describes a method in which a non-magnetic material is cast into a recess formed in a magnetic material, and thereafter, a part of the magnetic material is cut and removed to form a magnetic path forming member.
The magnetic path forming member is required to have a sufficient strength in order to fulfill a driving force transmitting function. In the case where the magnetic path forming member is formed by welding or casting, a joint between the magnetic material and the non-magnetic material needs to have a certain degree of thickness because the strength of the joint is lower than that of the magnetic material. Further, it is desired that a magnetic path forming member be manufactured at a cost lower than that at which a magnetic path forming member is manufactured by welding or casting.